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diff --git a/share/doc/papers/sysperf/2.t b/share/doc/papers/sysperf/2.t new file mode 100644 index 0000000..703cbb6 --- /dev/null +++ b/share/doc/papers/sysperf/2.t @@ -0,0 +1,258 @@ +.\" Copyright (c) 1985 The Regents of the University of California. +.\" All rights reserved. +.\" +.\" Redistribution and use in source and binary forms, with or without +.\" modification, are permitted provided that the following conditions +.\" are met: +.\" 1. Redistributions of source code must retain the above copyright +.\" notice, this list of conditions and the following disclaimer. +.\" 2. Redistributions in binary form must reproduce the above copyright +.\" notice, this list of conditions and the following disclaimer in the +.\" documentation and/or other materials provided with the distribution. +.\" 3. All advertising materials mentioning features or use of this software +.\" must display the following acknowledgement: +.\" This product includes software developed by the University of +.\" California, Berkeley and its contributors. +.\" 4. Neither the name of the University nor the names of its contributors +.\" may be used to endorse or promote products derived from this software +.\" without specific prior written permission. +.\" +.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND +.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE +.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE +.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE +.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL +.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS +.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) +.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT +.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY +.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF +.\" SUCH DAMAGE. +.\" +.\" @(#)2.t 5.1 (Berkeley) 4/17/91 +.\" +.ds RH Observation techniques +.NH +Observation techniques +.PP +There are many tools available for monitoring the performance +of the system. +Those that we found most useful are described below. +.NH 2 +System maintenance tools +.PP +Several standard maintenance programs are invaluable in +observing the basic actions of the system. +The \fIvmstat\fP(1) +program is designed to be an aid to monitoring +systemwide activity. Together with the +\fIps\fP\|(1) +command (as in ``ps av''), it can be used to investigate systemwide +virtual memory activity. +By running \fIvmstat\fP +when the system is active you can judge the system activity in several +dimensions: job distribution, virtual memory load, paging and swapping +activity, disk and cpu utilization. +Ideally, to have a balanced system in activity, +there should be few blocked (b) jobs, +there should be little paging or swapping activity, there should +be available bandwidth on the disk devices (most single arms peak +out at 25-35 tps in practice), and the user cpu utilization (us) should +be high (above 50%). +.PP +If the system is busy, then the count of active jobs may be large, +and several of these jobs may often be blocked (b). If the virtual +memory is active, then the paging demon will be running (sr will +be non-zero). It is healthy for the paging demon to free pages when +the virtual memory gets active; it is triggered by the amount of free +memory dropping below a threshold and increases its pace as free memory +goes to zero. +.PP +If you run \fIvmstat\fP +when the system is busy (a ``vmstat 5'' gives all the +numbers computed by the system), you can find +imbalances by noting abnormal job distributions. If many +processes are blocked (b), then the disk subsystem +is overloaded or imbalanced. If you have several non-dma +devices or open teletype lines that are ``ringing'', or user programs +that are doing high-speed non-buffered input/output, then the system +time may go high (60-80% or higher). +It is often possible to pin down the cause of high system time by +looking to see if there is excessive context switching (cs), interrupt +activity (in) or system call activity (sy). Long term measurements +on one of +our large machines show +an average of 60 context switches and interrupts +per second and an average of 90 system calls per second. +.PP +If the system is heavily loaded, or if you have little memory +for your load (1 megabyte is little in our environment), then the system +may be forced to swap. This is likely to be accompanied by a noticeable +reduction in the system responsiveness and long pauses when interactive +jobs such as editors swap out. +.PP +A second important program is \fIiostat\fP\|(1). +\fIIostat\fP +iteratively reports the number of characters read and written to terminals, +and, for each disk, the number of transfers per second, kilobytes +transferred per second, +and the milliseconds per average seek. +It also gives the percentage of time the system has +spent in user mode, in user mode running low priority (niced) processes, +in system mode, and idling. +.PP +To compute this information, for each disk, seeks and data transfer completions +and the number of words transferred are counted; +for terminals collectively, the number +of input and output characters are counted. +Also, every 100 ms, +the state of each disk is examined +and a tally is made if the disk is active. +From these numbers and the transfer rates +of the devices it is possible to determine +average seek times for each device. +.PP +When filesystems are poorly placed on the available +disks, figures reported by \fIiostat\fP can be used +to pinpoint bottlenecks. Under heavy system load, disk +traffic should be spread out among the drives with +higher traffic expected to the devices where the root, swap, and +/tmp filesystems are located. When multiple disk drives are +attached to the same controller, the system will +attempt to overlap seek operations with I/O transfers. When +seeks are performed, \fIiostat\fP will show +non-zero average seek times. Most modern disk drives should +exhibit an average seek time of 25-35 ms. +.PP +Terminal traffic reported by \fIiostat\fP should be heavily +output oriented unless terminal lines are being used for +data transfer by programs such as \fIuucp\fP. Input and +output rates are system specific. Screen editors +such as \fIvi\fP and \fIemacs\fP tend to exhibit output/input +ratios of anywhere from 5/1 to 8/1. On one of our largest +systems, 88 terminal lines plus 32 pseudo terminals, we observed +an average of 180 characters/second input and 450 characters/second +output over 4 days of operation. +.NH 2 +Kernel profiling +.PP +It is simple to build a 4.2BSD kernel that will automatically +collect profiling information as it operates simply by specifying the +.B \-p +option to \fIconfig\fP\|(8) when configuring a kernel. +The program counter sampling can be driven by the system clock, +or by an alternate real time clock. +The latter is highly recommended as use of the system clock results +in statistical anomalies in accounting for +the time spent in the kernel clock routine. +.PP +Once a profiling system has been booted statistic gathering is +handled by \fIkgmon\fP\|(8). +\fIKgmon\fP allows profiling to be started and stopped +and the internal state of the profiling buffers to be dumped. +\fIKgmon\fP can also be used to reset the state of the internal +buffers to allow multiple experiments to be run without +rebooting the machine. +.PP +The profiling data is processed with \fIgprof\fP\|(1) +to obtain information regarding the system's operation. +Profiled systems maintain histograms of the kernel program counter, +the number of invocations of each routine, +and a dynamic call graph of the executing system. +The postprocessing propagates the time spent in each +routine along the arcs of the call graph. +\fIGprof\fP then generates a listing for each routine in the kernel, +sorted according to the time it uses +including the time of its call graph descendents. +Below each routine entry is shown its (direct) call graph children, +and how their times are propagated to this routine. +A similar display above the routine shows how this routine's time and the +time of its descendents is propagated to its (direct) call graph parents. +.PP +A profiled system is about 5-10% larger in its text space because of +the calls to count the subroutine invocations. +When the system executes, +the profiling data is stored in a buffer that is 1.2 +times the size of the text space. +All the information is summarized in memory, +it is not necessary to have a trace file +being continuously dumped to disk. +The overhead for running a profiled system varies; +under normal load we see anywhere from 5-25% +of the system time spent in the profiling code. +Thus the system is noticeably slower than an unprofiled system, +yet is not so bad that it cannot be used in a production environment. +This is important since it allows us to gather data +in a real environment rather than trying to +devise synthetic work loads. +.NH 2 +Kernel tracing +.PP +The kernel can be configured to trace certain operations by +specifying ``options TRACE'' in the configuration file. This +forces the inclusion of code that records the occurrence of +events in \fItrace records\fP in a circular buffer in kernel +memory. Events may be enabled/disabled selectively while the +system is operating. Each trace record contains a time stamp +(taken from the VAX hardware time of day clock register), an +event identifier, and additional information that is interpreted +according to the event type. Buffer cache operations, such as +initiating a read, include +the disk drive, block number, and transfer size in the trace record. +Virtual memory operations, such as a pagein completing, include +the virtual address and process id in the trace record. The circular +buffer is normally configured to hold 256 16-byte trace records.\** +.FS +\** The standard trace facilities distributed with 4.2 +differ slightly from those described here. The time stamp in the +distributed system is calculated from the kernel's time of day +variable instead of the VAX hardware register, and the buffer cache +trace points do not record the transfer size. +.FE +.PP +Several user programs were written to sample and interpret the +tracing information. One program runs in the background and +periodically reads the circular buffer of trace records. The +trace information is compressed, in some instances interpreted +to generate additional information, and a summary is written to a +file. In addition, the sampling program can also record +information from other kernel data structures, such as those +interpreted by the \fIvmstat\fP program. Data written out to +a file is further buffered to minimize I/O load. +.PP +Once a trace log has been created, programs that compress +and interpret the data may be run to generate graphs showing the +data and relationships between traced events and +system load. +.PP +The trace package was used mainly to investigate the operation of +the file system buffer cache. The sampling program maintained a +history of read-ahead blocks and used the trace information to +calculate, for example, percentage of read-ahead blocks used. +.NH 2 +Benchmark programs +.PP +Benchmark programs were used in two ways. First, a suite of +programs was constructed to calculate the cost of certain basic +system operations. Operations such as system call overhead and +context switching time are critically important in evaluating the +overall performance of a system. Because of the drastic changes in +the system between 4.1BSD and 4.2BSD, it was important to verify +the overhead of these low level operations had not changed appreciably. +.PP +The second use of benchmarks was in exercising +suspected bottlenecks. +When we suspected a specific problem with the system, +a small benchmark program was written to repeatedly use +the facility. +While these benchmarks are not useful as a general tool +they can give quick feedback on whether a hypothesized +improvement is really having an effect. +It is important to realize that the only real assurance +that a change has a beneficial effect is through +long term measurements of general timesharing. +We have numerous examples where a benchmark program +suggests vast improvements while the change +in the long term system performance is negligible, +and conversely examples in which the benchmark program run more slowly, +but the long term system performance improves significantly. |
